Chapter 9 Batch compilation (ocamlc)

This chapter describes the OCaml batch compiler ocamlc,
which compiles OCaml source files to bytecode object files and links
these object files to produce standalone bytecode executable files.
These executable files are then run by the bytecode interpreter
ocamlrun.

9.1 Overview of the compiler

The ocamlc command has a command-line interface similar to the one of
most C compilers. It accepts several types of arguments and processes them
sequentially, after all options have been processed:

Arguments ending in .mli are taken to be source files for
compilation unit interfaces. Interfaces specify the names exported by
compilation units: they declare value names with their types, define
public data types, declare abstract data types, and so on. From the
file x.mli, the ocamlc compiler produces a compiled interface
in the file x.cmi.

Arguments ending in .ml are taken to be source files for compilation
unit implementations. Implementations provide definitions for the
names exported by the unit, and also contain expressions to be
evaluated for their side-effects. From the file x.ml, the ocamlc
compiler produces compiled object bytecode in the file x.cmo.

If the interface file x.mli exists, the implementation
x.ml is checked against the corresponding compiled interface
x.cmi, which is assumed to exist. If no interface
x.mli is provided, the compilation of x.ml produces a
compiled interface file x.cmi in addition to the compiled
object code file x.cmo. The file x.cmi produced
corresponds to an interface that exports everything that is defined in
the implementation x.ml.

Arguments ending in .cmo are taken to be compiled object bytecode. These
files are linked together, along with the object files obtained
by compiling .ml arguments (if any), and the OCaml standard
library, to produce a standalone executable program. The order in
which .cmo and .ml arguments are presented on the command line is
relevant: compilation units are initialized in that order at
run-time, and it is a link-time error to use a component of a unit
before having initialized it. Hence, a given x.cmo file must come
before all .cmo files that refer to the unit x.

Arguments ending in .cma are taken to be libraries of object bytecode.
A library of object bytecode packs in a single file a set of object
bytecode files (.cmo files). Libraries are built with ocamlc -a
(see the description of the -a option below). The object files
contained in the library are linked as regular .cmo files (see
above), in the order specified when the .cma file was built. The
only difference is that if an object file contained in a library is
not referenced anywhere in the program, then it is not linked in.

Arguments ending in .c are passed to the C compiler, which generates
a .o object file (.obj under Windows). This object file is linked
with the program if the -custom flag is set (see the description of
-custom below).

Arguments ending in .o or .a (.obj or .lib under Windows)
are assumed to be C object files and libraries. They are passed to the
C linker when linking in -custom mode (see the description of
-custom below).

Arguments ending in .so (.dll under Windows)
are assumed to be C shared libraries (DLLs). During linking, they are
searched for external C functions referenced from the OCaml code,
and their names are written in the generated bytecode executable.
The run-time system ocamlrun then loads them dynamically at program
start-up time.

The output of the linking phase is a file containing compiled bytecode
that can be executed by the OCaml bytecode interpreter:
the command named ocamlrun. If a.out is the name of the file
produced by the linking phase, the command

ocamlrun a.out arg1arg2 … argn

executes the compiled code contained in a.out, passing it as
arguments the character strings arg1 to argn.
(See chapter 11 for more details.)

On most systems, the file produced by the linking
phase can be run directly, as in:

./a.out arg1arg2 … argn

The produced file has the executable bit set, and it manages to launch
the bytecode interpreter by itself.

9.2 Options

The following command-line options are recognized by ocamlc.
The options -pack, -a, -c and -output-obj are mutually exclusive.

-a

Build a library(.cma file)
with the object files ( .cmo files)
given on the command line, instead of linking them into an executable file.
The name of the library must be set with the -o option.

If -custom, -cclib or -ccopt options are passed on the command
line, these options are stored in the resulting .cmalibrary. Then,
linking with this library automatically adds back the -custom,
-cclib and -ccopt options as if they had been provided on the
command line, unless the -noautolink option is given.

-absname

Force error messages to show absolute paths for file names.

-annot

Dump detailed information about the compilation (types, bindings,
tail-calls, etc). The information for file src.ml
is put into file src.annot. In case of a type error, dump
all the information inferred by the type-checker before the error.
The src.annot file can be used with the emacs commands given in
emacs/caml-types.el to display types and other annotations
interactively.

Read additional null character terminated command line arguments from filename.

-bin-annot

Dump detailed information about the compilation (types, bindings,
tail-calls, etc) in binary format. The information for file src.ml
(resp. src.mli) is put into file src.cmt
(resp. src.cmti). In case of a type error, dump
all the information inferred by the type-checker before the error.
The *.cmt and *.cmti files produced by -bin-annot contain
more information and are much more compact than the files produced by
-annot.

-c

Compile only. Suppress the linking phase of the
compilation. Source code files are turned into compiled files, but no
executable file is produced. This option is useful to
compile modules separately.

-ccccomp

Use ccomp as the C linker
when linking in “custom runtime” mode (see the -custom option)
and as the C compiler for compiling .c source files.

-cclib-llibname

Pass the -llibname option to the C linker
when linking in “custom runtime” mode (see the -custom option).
This causes the given C library to be linked with the program.

-ccoptoption

Pass the given option to the C compiler and linker.
When linking in “custom runtime” mode, for instance-ccopt -Ldir causes the C linker to search for C libraries in
directory dir.(See the -custom option.)

-colormode

Enable or disable colors in compiler messages (especially warnings and errors).
The following modes are supported:

auto

use heuristics to enable colors only if the output supports them (an ANSI-compatible tty terminal);

always

enable colors unconditionally;

never

disable color output.

The default setting is ’auto’, and the current heuristic
checks that the TERM environment variable exists and is
not empty or dumb, and that ’isatty(stderr)’ holds.

The environment variable OCAML_COLOR is considered if -color is not
provided. Its values are auto/always/never as above.

-compat-32

Check that the generated bytecode executable can run on 32-bit
platforms and signal an error if it cannot. This is useful when
compiling bytecode on a 64-bit machine.

-config

Print the version number of ocamlc and a detailed
summary of its configuration, then exit.

-custom

Link in “custom runtime” mode. In the default linking mode, the
linker produces bytecode that is intended to be executed with the
shared runtime system, ocamlrun. In the custom runtime mode, the
linker produces an output file that contains both the runtime system
and the bytecode for the program. The resulting file is larger, but it
can be executed directly, even if the ocamlrun command is not
installed. Moreover, the “custom runtime” mode enables static
linking of OCaml code with user-defined C functions, as described in
chapter 20.

Unix:
Never use the strip command on executables produced by ocamlc -custom,
this would remove the bytecode part of the executable.

Unix:
Security warning: never set the “setuid” or “setgid” bits on executables
produced by ocamlc -custom, this would make them vulnerable to attacks.

-dependocamldep-args

Compute dependencies, as the ocamldep command would do. The remaining
arguments are interpreted as if they were given to the ocamldep command.

-dllib-llibname

Arrange for the C shared library dlllibname.so
(dlllibname.dll under Windows) to be loaded dynamically
by the run-time system ocamlrun at program start-up time.

-dllpathdir

Adds the directory dir to the run-time search path for shared
C libraries. At link-time, shared libraries are searched in the
standard search path (the one corresponding to the -I option).
The -dllpath option simply stores dir in the produced
executable file, where ocamlrun can find it and use it as
described in section 11.3.

-for-packmodule-path

Generate an object file (.cmo)
that can later be included
as a sub-module (with the given access path) of a compilation unit
constructed with -pack. For instance,
ocamlc -for-pack P -c A.ml
will generate a..cmo that can
later be used with ocamlc -pack -o P.cmo a.cmo.
Note: you can still pack a module that was compiled without
-for-pack but in this case exceptions will be printed with the wrong
names.

-g

Add debugging information while compiling and linking. This option is
required in order to be able to debug the program with ocamldebug
(see chapter 17), and to produce stack backtraces when
the program terminates on an uncaught exception (see
section 11.2).

-i

Cause the compiler to print all defined names (with their inferred
types or their definitions) when compiling an implementation (.ml
file). No compiled files (.cmo and .cmi files) are produced.
This can be useful to check the types inferred by the
compiler. Also, since the output follows the syntax of interfaces, it
can help in writing an explicit interface (.mli file) for a file:
just redirect the standard output of the compiler to a .mli file,
and edit that file to remove all declarations of unexported names.

-Idirectory

Add the given directory to the list of directories searched for
compiled interface files (.cmi), compiled object code files .cmo,
libraries (.cma) and C libraries specified with -cclib -lxxx.
By default, the current directory is searched first, then the standard
library directory. Directories added with -I are searched after the
current directory, in the order in which they were given on the command line,
but before the standard library directory. See also option -nostdlib.

If the given directory starts with +, it is taken relative to the
standard library directory. For instance, -I +unix adds the
subdirectory unix of the standard library to the search path.

-implfilename

Compile the file filename as an implementation file, even if its
extension is not .ml.

-intffilename

Compile the file filename as an interface file, even if its
extension is not .mli.

Labels are not ignored in types, labels may be used in applications,
and labelled parameters can be given in any order. This is the default.

-linkall

Force all modules contained in libraries to be linked in. If this
flag is not given, unreferenced modules are not linked in. When
building a library (option -a), setting the -linkall option forces all
subsequent links of programs involving that library to link all the
modules contained in the library. When compiling a module (option
-c), setting the -linkall option ensures that this module will
always be linked if it is put in a library and this library is linked.

-make-runtime

Build a custom runtime system (in the file specified by option -o)
incorporating the C object files and libraries given on the command
line. This custom runtime system can be used later to execute
bytecode executables produced with the
ocamlc -use-runtimeruntime-name option.
See section 20.1.6 for more information.

-no-alias-deps

Do not record dependencies for module aliases. See
section 8.12 for more information.

-no-app-funct

Deactivates the applicative behaviour of functors. With this option,
each functor application generates new types in its result and
applying the same functor twice to the same argument yields two
incompatible structures.

-noassert

Do not compile assertion checks. Note that the special form
assert false is always compiled because it is typed specially.
This flag has no effect when linking already-compiled files.

-noautolink

When linking .cmalibraries, ignore -custom, -cclib and -ccopt
options potentially contained in the libraries (if these options were
given when building the libraries). This can be useful if a library
contains incorrect specifications of C libraries or C options; in this
case, during linking, set -noautolink and pass the correct C
libraries and options on the command line.

-nolabels

Ignore non-optional labels in types. Labels cannot be used in
applications, and parameter order becomes strict.

-nostdlib

Do not include the standard library directory in the list of
directories searched for
compiled interface files (.cmi), compiled object code files
(.cmo), libraries (.cma), and C libraries specified with
-cclib -lxxx. See also option -I.

-oexec-file

Specify the name of the output file produced by the
compiler. The
default output name is a.out under Unix and camlprog.exe under
Windows. If the -a option is given, specify the name of the library
produced. If the -pack option is given, specify the name of the
packed object file produced. If the -output-obj option is given,
specify the name of the output file produced.
If the -c option is given, specify the name of the object
file produced for the next source file that appears on the
command line.

-opaque

When the native compiler compiles an implementation, by default it
produces a .cmx file containing information for cross-module
optimization. It also expects .cmx files to be present for the
dependencies of the currently compiled source, and uses them for
optimization. Since OCaml 4.03, the compiler will emit a warning if it
is unable to locate the .cmx file of one of those dependencies.

The -opaque option, available since 4.04, disables cross-module
optimization information for the currently compiled unit. When
compiling .mli interface, using -opaque marks the compiled .cmi
interface so that subsequent compilations of modules that depend on it
will not rely on the corresponding .cmx file, nor warn if it is
absent. When the native compiler compiles a .ml implementation,
using -opaque generates a .cmx that does not contain any
cross-module optimization information.

Using this option may degrade the quality of generated code, but it
reduces compilation time, both on clean and incremental
builds. Indeed, with the native compiler, when the implementation of
a compilation unit changes, all the units that depend on it may need
to be recompiled – because the cross-module information may have
changed. If the compilation unit whose implementation changed was
compiled with -opaque, no such recompilation needs to occur. This
option can thus be used, for example, to get faster edit-compile-test
feedback loops.

-openModule

Opens the given module before processing the interface or
implementation files. If several -open options are given,
they are processed in order, just as if
the statements open!Module1;;...open!ModuleN;;
were added at the top of each file.

-output-obj

Cause the linker to produce a C object file instead of
a bytecode executable file.
This is useful to wrap OCaml code as a C library,
callable from any C program. See chapter 20,
section 20.7.5. The name of the output object file
must be set with the -o option.
This option can also be used to produce a C source file (.c extension) or
a compiled shared/dynamic library (.so extension, .dll under Windows).

-pack

Build a bytecode object file (.cmo file) and its associated compiled
interface (.cmi) that combines the object
files given on the command line, making them appear as sub-modules of
the output .cmo file. The name of the output .cmo file must be
given with the -o option. For instance,

ocamlc -pack -o p.cmo a.cmo b.cmo c.cmo

generates compiled files p.cmo and p.cmi describing a compilation
unit having three sub-modules A, B and C, corresponding to the
contents of the object files a.cmo, b.cmo and c.cmo. These
contents can be referenced as P.A, P.B and P.C in the remainder
of the program.

-pluginplugin

Dynamically load the code of the given plugin
(a .cmo, .cma or .cmxs file) in the compiler. plugin must exist in
the same kind of code as the compiler (ocamlc.byte must load bytecode
plugins, while ocamlc.opt must load native code plugins), and
extension adaptation is done automatically for .cma files (to .cmxs files
if the compiler is compiled in native code).

-ppcommand

Cause the compiler to call the given command as a preprocessor
for each source file. The output of command is redirected to
an intermediate file, which is compiled. If there are no compilation
errors, the intermediate file is deleted afterwards.

-ppxcommand

After parsing, pipe the abstract syntax tree through the preprocessor
command. The module Ast_mapper, described in
chapter 27: Ast_mapper
,
implements the external interface of a preprocessor.

-principal

Check information path during type-checking, to make sure that all
types are derived in a principal way. When using labelled arguments
and/or polymorphic methods, this flag is required to ensure future
versions of the compiler will be able to infer types correctly, even
if internal algorithms change.
All programs accepted in -principal mode are also accepted in the
default mode with equivalent types, but different binary signatures,
and this may slow down type checking; yet it is a good idea to
use it once before publishing source code.

-rectypes

Allow arbitrary recursive types during type-checking. By default,
only recursive types where the recursion goes through an object type
are supported.Note that once you have created an interface using this
flag, you must use it again for all dependencies.

-runtime-variantsuffix

Add the suffix string to the name of the runtime library used by
the program. Currently, only one such suffix is supported: d, and
only if the OCaml compiler was configured with option
-with-debug-runtime. This suffix gives the debug version of the
runtime, which is useful for debugging pointer problems in low-level
code such as C stubs.

-safe-string

Enforce the separation between types string and bytes,
thereby making strings read-only. This is the default.

-short-paths

When a type is visible under several module-paths, use the shortest
one when printing the type’s name in inferred interfaces and error and
warning messages. Identifier names starting with an underscore _ or
containing double underscores __ incur a penalty of +10 when computing
their length.

-strict-sequence

Force the left-hand part of each sequence to have type unit.

-strict-formats

Reject invalid formats that were accepted in legacy format
implementations. You should use this flag to detect and fix such
invalid formats, as they will be rejected by future OCaml versions.

-unboxed-types

When a type is unboxable (i.e. a record with a single argument or a
concrete datatype with a single constructor of one argument) it will
be unboxed unless annotated with [@@ocaml.boxed].

-no-unboxed-types

When a type is unboxable it will be boxed unless annotated with
[@@ocaml.unboxed]. This is the default.

-unsafe

Turn bound checking off for array and string accesses (the v.(i) and
s.[i] constructs). Programs compiled with -unsafe are therefore
slightly faster, but unsafe: anything can happen if the program
accesses an array or string outside of its bounds.
Additionally, turn off the check for zero divisor in integer division
and modulus operations. With -unsafe, an integer division
(or modulus) by zero can halt the program or continue with an
unspecified result instead of raising a Division_by_zero exception.

-unsafe-string

Identify the types string and bytes, thereby making strings writable.
This is intended for compatibility with old source code and should not
be used with new software.

-use-runtimeruntime-name

Generate a bytecode executable file that can be executed on the custom
runtime system runtime-name, built earlier with
ocamlc -make-runtimeruntime-name.
See section 20.1.6 for more information.

-v

Print the version number of the compiler and the location of the
standard library directory, then exit.

-verbose

Print all external commands before they are executed,
in particular invocations of the C compiler and linker in -custom mode.
Useful to debug C library problems.

-vmthread

Compile or link multithreaded programs, in combination with the
VM-level threads library described in chapter 31.

-version or -vnum

Print the version number of the compiler in short form (e.g. 3.11.0),
then exit.

-wwarning-list

Enable, disable, or mark as fatal the warnings specified by the argument
warning-list.
Each warning can be enabled or disabled, and each warning
can be fatal or non-fatal.
If a warning is disabled, it isn’t displayed and doesn’t affect
compilation in any way (even if it is fatal). If a warning is
enabled, it is displayed normally by the compiler whenever the source
code triggers it. If it is enabled and fatal, the compiler will also
stop with an error after displaying it.

The warning-list argument is a sequence of warning specifiers,
with no separators between them. A warning specifier is one of the
following:

+num

Enable warning number num.

-num

Disable warning number num.

@num

Enable and mark as fatal warning number num.

+num1..num2

Enable warnings in the given range.

-num1..num2

Disable warnings in the given range.

@num1..num2

Enable and mark as fatal warnings in
the given range.

+letter

Enable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.

-letter

Disable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.

@letter

Enable and mark as fatal the set of warnings
corresponding to letter. The letter may be uppercase or
lowercase.

uppercase-letter

Enable the set of warnings corresponding
to uppercase-letter.

lowercase-letter

Disable the set of warnings corresponding
to lowercase-letter.

Warning numbers and letters which are out of the range of warnings
that are currently defined are ignored. The warnings are as follows.

1

Suspicious-looking start-of-comment mark.

2

Suspicious-looking end-of-comment mark.

3

Deprecated feature.

4

Fragile pattern matching: matching that will remain complete even
if additional constructors are added to one of the variant types
matched.

5

Partially applied function: expression whose result has function
type and is ignored.

6

Label omitted in function application.

7

Method overridden.

8

Partial match: missing cases in pattern-matching.

9

Missing fields in a record pattern.

10

Expression on the left-hand side of a sequence that doesn’t have type
unit (and that is not a function, see warning number 5).

11

Redundant case in a pattern matching (unused match case).

12

Redundant sub-pattern in a pattern-matching.

13

Instance variable overridden.

14

Illegal backslash escape in a string constant.

15

Private method made public implicitly.

16

Unerasable optional argument.

17

Undeclared virtual method.

18

Non-principal type.

19

Type without principality.

20

Unused function argument.

21

Non-returning statement.

22

Preprocessor warning.

23

Useless record with clause.

24

Bad module name: the source file name is not a valid OCaml module name.

25

Deprecated: now part of warning 8.

26

Suspicious unused variable: unused variable that is bound
with let or as, and doesn’t start with an underscore (_)
character.

27

Innocuous unused variable: unused variable that is not bound with
let nor as, and doesn’t start with an underscore (_)
character.

28

Wildcard pattern given as argument to a constant constructor.

29

Unescaped end-of-line in a string constant (non-portable code).

30

Two labels or constructors of the same name are defined in two
mutually recursive types.

The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60.
It is displayed by ocamlc -help.
Note that warnings 5 and 10 are not always triggered, depending on
the internals of the type checker.

-warn-errorwarning-list

Mark as fatal the warnings specified in the argument warning-list.
The compiler will stop with an error when one of these warnings is
emitted. The warning-list has the same meaning as for
the -w option: a + sign (or an uppercase letter) marks the
corresponding warnings as fatal, a -
sign (or a lowercase letter) turns them back into non-fatal warnings,
and a @ sign both enables and marks as fatal the corresponding
warnings.

Note: it is not recommended to use warning sets (i.e. letters) as
arguments to -warn-error
in production code, because this can break your build when future versions
of OCaml add some new warnings.

The default setting is -warn-error -a+31 (only warning 31 is fatal).

-warn-help

Show the description of all available warning numbers.

-where

Print the location of the standard library, then exit.

-file

Process file as a file name, even if it starts with a dash (-)
character.

-help or --help

Display a short usage summary and exit.

Contextual control of command-line options

The compiler command line can be modified “from the outside”
with the following mechanisms. These are experimental
and subject to change. They should be used only for experimental and
development work, not in released packages.

OCAMLPARAM (environment variable)

A set of arguments that will be inserted before or after the arguments from
the command line. Arguments are specified in a comma-separated list
of name=value pairs. A _ is used to specify the position of
the command line arguments, i.e. a=x,_,b=y means that a=x should be
executed before parsing the arguments, and b=y after. Finally,
an alternative separator can be specified as the
first character of the string, within the set :|; ,.

ocaml_compiler_internal_params (file in the stdlib directory)

A mapping of file names to lists of arguments that
will be added to the command line (and OCAMLPARAM) arguments.

OCAML_FLEXLINK (environment variable)

Alternative executable to use on native
Windows for flexlink instead of the
configured value. Primarily used for bootstrapping.

9.3 Modules and the file system

This short section is intended to clarify the relationship between the
names of the modules corresponding to compilation units and the names
of the files that contain their compiled interface and compiled
implementation.

The compiler always derives the module name by taking the capitalized
base name of the source file (.ml or .mli file). That is, it
strips the leading directory name, if any, as well as the .ml or
.mli suffix; then, it set the first letter to uppercase, in order to
comply with the requirement that module names must be capitalized.
For instance, compiling the file mylib/misc.ml provides an
implementation for the module named Misc. Other compilation units
may refer to components defined in mylib/misc.ml under the names
Misc.name; they can also do open Misc, then use unqualified
names name.

The .cmi and .cmo files produced by the compiler have the same
base name as the source file. Hence, the compiled files always have
their base name equal (modulo capitalization of the first letter) to
the name of the module they describe (for .cmi files) or implement
(for .cmo files).

When the compiler encounters a reference to a free module identifier
Mod, it looks in the search path for a file named Mod.cmi or mod.cmi
and loads the compiled interface
contained in that file. As a consequence, renaming .cmi files is not
advised: the name of a .cmi file must always correspond to the name
of the compilation unit it implements. It is admissible to move them
to another directory, if their base name is preserved, and the correct
-I options are given to the compiler. The compiler will flag an
error if it loads a .cmi file that has been renamed.

Compiled bytecode files (.cmo files), on the other hand, can be
freely renamed once created. That’s because the linker never attempts
to find by itself the .cmo file that implements a module with a
given name: it relies instead on the user providing the list of .cmo
files by hand.

9.4 Common errors

This section describes and explains the most frequently encountered
error messages.

Cannot find file filename

The named file could not be found in the current directory, nor in the
directories of the search path. The filename is either a
compiled interface file (.cmi file), or a compiled bytecode file
(.cmo file). If filename has the format mod.cmi, this
means you are trying to compile a file that references identifiers
from module mod, but you have not yet compiled an interface for
module mod. Fix: compile mod.mli or mod.ml
first, to create the compiled interface mod.cmi.

If filename has the format mod.cmo, this
means you are trying to link a bytecode object file that does not
exist yet. Fix: compile mod.ml first.

If your program spans several directories, this error can also appear
because you haven’t specified the directories to look into. Fix: add
the correct -I options to the command line.

Corrupted compiled interface filename

The compiler produces this error when it tries to read a compiled
interface file (.cmi file) that has the wrong structure. This means
something went wrong when this .cmi file was written: the disk was
full, the compiler was interrupted in the middle of the file creation,
and so on. This error can also appear if a .cmi file is modified after
its creation by the compiler. Fix: remove the corrupted .cmi file,
and rebuild it.

This expression has type t1, but is used with type t2

This is by far the most common type error in programs. Type t1 is
the type inferred for the expression (the part of the program that is
displayed in the error message), by looking at the expression itself.
Type t2 is the type expected by the context of the expression; it
is deduced by looking at how the value of this expression is used in
the rest of the program. If the two types t1 and t2 are not
compatible, then the error above is produced.

In some cases, it is hard to understand why the two types t1 and
t2 are incompatible. For instance, the compiler can report that
“expression of type foo cannot be used with type foo”, and it
really seems that the two types foo are compatible. This is not
always true. Two type constructors can have the same name, but
actually represent different types. This can happen if a type
constructor is redefined. Example:

This result in the error message “expression C of type foo cannot
be used with type foo”.

The type of this expression, t, contains type variables
that cannot be generalized

Type variables ('a, 'b, …) in a type t can be in either
of two states: generalized (which means that the type t is valid
for all possible instantiations of the variables) and not generalized
(which means that the type t is valid only for one instantiation
of the variables). In a let binding let name = expr,
the type-checker normally generalizes as many type variables as
possible in the type of expr. However, this leads to unsoundness
(a well-typed program can crash) in conjunction with polymorphic
mutable data structures. To avoid this, generalization is performed at
let bindings only if the bound expression expr belongs to the
class of “syntactic values”, which includes constants, identifiers,
functions, tuples of syntactic values, etc. In all other cases (for
instance, expr is a function application), a polymorphic mutable
could have been created and generalization is therefore turned off for
all variables occurring in contravariant or non-variant branches of the
type. For instance, if the type of a non-value is 'a list the
variable is generalizable (list is a covariant type constructor),
but not in 'a list -> 'a list (the left branch of -> is
contravariant) or 'a ref (ref is non-variant).

Non-generalized type variables in a type cause no difficulties inside
a given structure or compilation unit (the contents of a .ml file,
or an interactive session), but they cannot be allowed inside
signatures nor in compiled interfaces (.cmi file), because they
could be used inconsistently later. Therefore, the compiler
flags an error when a structure or compilation unit defines a value
name whose type contains non-generalized type variables. There
are two ways to fix this error:

Add a type constraint or a .mli file to give a monomorphic
type (without type variables) to name. For instance, instead of
writing

This error appears when trying to link an incomplete or incorrectly
ordered set of files. Either you have forgotten to provide an
implementation for the compilation unit named mod on the command line
(typically, the file named mod.cmo, or a library containing
that file). Fix: add the missing .ml or .cmo file to the command
line. Or, you have provided an implementation for the module named
mod, but it comes too late on the command line: the
implementation of mod must come before all bytecode object files
that reference mod. Fix: change the order of .ml and .cmo
files on the command line.

Of course, you will always encounter this error if you have mutually
recursive functions across modules. That is, function Mod1.f calls
function Mod2.g, and function Mod2.g calls function Mod1.f.
In this case, no matter what permutations you perform on the command
line, the program will be rejected at link-time. Fixes:

This error appears when trying to link code that calls external
functions written in C. As explained in
chapter 20, such code must be linked with C libraries that
implement the required f C function. If the C libraries in
question are not shared libraries (DLLs), the code must be linked in
“custom runtime” mode. Fix: add the required C libraries to the
command line, and possibly the -custom option.

9.5 Warning reference

This section describes and explains in detail some warnings:

9.5.1 Warning 9: missing fields in a record pattern

When pattern matching on records, it can be useful to match only few
fields of a record. Eliding fields can be done either implicitly
or explicitly by ending the record pattern with ; _.
However, implicit field elision is at odd with pattern matching
exhaustiveness checks.
Enabling warning 9 prioritizes exhaustiveness checks over the
convenience of implicit field elision and will warn on implicit
field elision in record patterns. In particular, this warning can
help to spot exhaustive record pattern that may need to be updated
after the addition of new fields to a record type.

9.5.2 Warning 52: fragile constant pattern

Some constructors, such as the exception constructors Failure and
Invalid_argument, take as parameter a string value holding
a text message intended for the user.

These text messages are usually not stable over time: call sites
building these constructors may refine the message in a future
version to make it more explicit, etc. Therefore, it is dangerous to
match over the precise value of the message. For example, until
OCaml 4.02, Array.iter2 would raise the exception

Invalid_argument "arrays must have the same length"

Since 4.03 it raises the more helpful message

Invalid_argument "Array.iter2: arrays must have the same length"

but this means that any code of the form

try ...
with Invalid_argument "arrays must have the same length" -> ...

is now broken and may suffer from uncaught exceptions.

Warning 52 is there to prevent users from writing such fragile code
in the first place. It does not occur on every matching on a literal
string, but only in the case in which library authors expressed
their intent to possibly change the constructor parameter value in
the future, by using the attribute ocaml.warn_on_literal_pattern
(see the manual section on builtin attributes in
8.16.1):

In particular, all built-in exceptions with a string argument have
this attribute set: Invalid_argument, Failure, Sys_error will
all raise this warning if you match for a specific string argument.

If your code raises this warning, you should not change the
way you test for the specific string to avoid the warning (for
example using a string equality inside the right-hand-side instead
of a literal pattern), as your code would remain fragile. You should
instead enlarge the scope of the pattern by matching on all possible
values.

let warning = function
| Foo _ -> 0
| _ -> 1

This may require some care: if the scrutinee may return several
different cases of the same pattern, or raise distinct instances of
the same exception, you may need to modify your code to separate
those several cases.

The only case where that transformation is not possible is if a given
function call may raise distinct exceptions with the same constructor
but different string values. In this case, you will have to check for
specific string values. This is dangerous API design and it should be
discouraged: it’s better to define more precise exception constructors
than store useful information in strings.

9.5.3 Warning 57: Ambiguous or-pattern variables under guard

The semantics of or-patterns in OCaml is specified with
a left-to-right bias: a value v matches the pattern p|q
if it matches p or q, but if it matches both,
the environment captured by the match is the environment captured by
p, never the one captured by q.

While this property is generally intuitive, there is at least one specific
case where a different semantics might be expected.
Consider a pattern followed by a when-guard:
|pwheng->e, for example:

| ((Const x, _) | (_, Const x)) when is_neutral x -> branch

The semantics is clear:
match the scrutinee against the pattern, if it matches, test the guard,
and if the guard passes, take the branch.
In particular, consider the input (Consta, Constb), where
a fails the test is_neutrala, while b passes the test
is_neutralb. With the left-to-right semantics, the clause above is
not taken by its input: matching (Consta, Constb)
against the or-pattern succeeds in the left branch, it returns the
environment x->a, and then the guard
is_neutrala is tested and fails, the branch is not taken.

However, another semantics may be considered more natural here:
any pair that has one side passing the test will take the branch. With this
semantics the previous code fragment would be equivalent to

Warning 57 is dedicated to these confusing cases where the
specified left-to-right semantics is not equivalent to a non-deterministic
semantics (any branch can be taken) relatively to a specific guard.
More precisely, it warns when guard uses “ambiguous” variables, that are bound
to different parts of the scrutinees by different sides of a or-pattern.